Epub ipad compatability

I recently created an epub3 book for download. It is meant to be compatable with the ipad.

People are able to download it to ibooks on my macbook with ease.

Yet anyone who tries to download it directly to their ipad (via Safari) is not able to.

I recommend that they update to the latest ibooks software. Still makes no difference. The only way they seem able to download it is if they download it to their computer and then transfer it to their ipad!

I can also get the epub 3 on to the ipad via the dropbox app.

I really need to be able to get it to work with the ipad via a safari download. Does anyone have a clue where I am going wrong?

It appears there was some sort of bug with the ios 9 release. If you google around a bit you'll see there are a number of reports of the problem. One solution that appears to have worked for some people is to reset their settings, but it looks like it's out of your control to do anything about.